Development of the trochlear nerve in the human embryos.

Investigations were made on serial sections of human embryos at developmental stages from 13 to 23 (32-56 postovulatory days). The trochlear nucleus appears in the posterior region of the basal plate of the midbrain at stage 13. It is composed of large neurons, the processes of which are sharply defined. Since stage 15 the trochlear fibers can be followed through the alar region to their decussation in the mid-dorsal part of the midbrain. The trochlear nerve emerges from the dorsal surface of the lowest part of the midbrain.